Hydraulic actuators are used in a wide variety of industrial applications. One of the more common uses is as actuators on work vehicles. Work vehicles, such as agricultural tractors, road graders, telehandlers, skid steer loaders, and mobile drilling rigs, use either single- or double-acting hydraulic actuators to move various components of the work vehicle and to move implements attached to the work vehicle with respect to the vehicle and with respect to each other.
A common method of manufacturing these actuators is to machine and polish the inside of a tube, such as a cylindrical tube. A plug or end cap is machined to enclose one end of the tube through which fluid will be introduced or removed from the actuator. The plug is partially inserted into the cylindrical tube, clamped in a rotational welding machine, and rotated in that machine while a circumferential weld is made that bonds one end of the tube to a portion of the plug.
To ensure that the plug and the cylindrical tube are properly aligned during the welding process, the plug is usually provided with a pilot portion on one end that is inserted into the tube. This pilot portion has a smaller diameter than the remainder of the plug portion and the junction between the pilot portion and the remainder is formed as a planar or conical shoulder. To assemble the tube and plug, the pilot portion is inserted into the tube until the shoulder on the plug abuts an end face of the tube. The weld is formed between the end face of the tube and the abutting shoulder portion of the plug.
A common failure mode for such welded actuators is that of weld failure. Hydraulic pressure acting against the inside surface of the tube creates hoop stress, which tends to cause the tube to expand or increase in diameter. The plug, on the other hand, is typically made of a rigid, solid piece of steel that does not expand when hydraulic fluid presses against its internal surfaces. As a result, a bending stress is created right at the weld joint coupling the tube and the plug. The tube expands radially when pressure is applied. The plug does not expand. Since the junction between the tube and the plug is the circumferential weld joint, it is the circumferential weld joint where the stress is at a maximum.
One way of avoiding failures at the tube-to-plug joint has been to provide a more flexible coupling. For example, rather than employing a weld to join the tube and plug, many actuators, especially smaller actuators, use a thread joint between the tube and plug. In these actuators, a pilot portion of the outside diameter of the plug is threaded, and a corresponding inside portion of the end of the tube is also threaded. To couple the two together, the threads on the outside of the plug are engaged with the threads on the inside of the tube and the two are threaded together. When hydraulic fluid under pressure is introduced into the actuator, the tube expands slightly due to the hoop stress generated by the fluid. Since the bond between the tube and the plug is a thread joint, the tube is free to expand slightly thereby slightly increasing the gap between the tube and the plug. This non-restrictive joint allows slight expansion of the tube to occur without additional stresses of a joint trying to restrain it. In this manner, the tube is made stronger. In addition, by eliminating the weld joint, the “cast” portion of the actuator, the actuator is made much more resistant to stress generally.
Of course, since the tube is permitted to expand with respect to the plug, a gap between the two along the thread joint is created. This gap, although small, provides a fluid leakage path. Fluid inside the actuator will leak out of the actuator along this thread joint. For this reason, a fluid tight seal that is relatively flexible is placed between the plug and the tube. In smaller actuators, this may be nothing more than a wrapping of thin Teflon® tape around the external threads on the plug. For larger actuators, however, such as those that have an area greater than about ½″ in diameter, seal, such as an O-ring, is typically placed in a circumferential or otherwise peripheral groove in the plug before it is inserted into the tube. The O-ring extends circumferentially around the diameter or perimeter of the plug and abuts both the plug and the tube providing a generally fluid-tight seal between the two that prevents fluid in the actuator from leaking out between the threads on the plug and the mating threads on the tube. When the tube in these threaded cap arrangements are pressurized with hydraulic fluid, it expands. The O-ring, however, is selected to be sufficiently pre-loaded to maintain contact with the internal walls of the tube even when it expands slightly because of hoop stress.
Nevertheless, a problem with threaded actuators is that forming the threads is an expensive operation relative to welding. Roger Mickelson recognized a need in the art for a welded actuator that reduced bending stress, and disclosed an improved hydraulic actuator that provides the low cost and ease of manufacture of a welded actuator yet reduces the longitudinal tensile forces on the weld to increase the actuator's longevity in U.S. Pat. No. 6,637,315, issued Oct. 28, 2003, and U.S. patent application Ser. No. 10/037,405, filed Dec. 21, 2001. U.S. Pat. No. 6,637,315 and U.S. patent application Ser. No. 10/037,405 are incorporated herein by reference.
A problem presented by inserting a seal into an actuator having a welded plug is that the seal cannot be placed into the tube along with the plug, as in an actuator having a threaded connection between the plug and the tube. Rather, to prevent damage to the seal during the welding operation, the seal must be inserted after the plug is welded to the tube. Therefore, even with the advances in the art represented by U.S. Pat. No. 6,637,315 and U.S. patent application Ser. No. 10/037,405, there is a need for an apparatus and method that make assembly of the hydraulic actuator having a welded tube and plug simpler and therefore less costly.